This invention relates to a refrigerant system of the type typically used in refrigerated display cases as for frozen foods or the like, and more particularly relates to a refrigeration system employing a multiple valve arrangement upstream of the evaporator.
Efficient marketing of frozen-food products since the early 1960's has contributed significantly to the general health and welfare of the consumer. One substantial factor enabling the efficient marketing of frozen-food products has been the wide-spread adoption of refrigerated display cases by grocery stores, with the most popular in recent years being the open front, multiple air-curtain type. These display cases are usually so-called "low temperature" cases for frozen foods, and "normal temperature" cases for meat, dairy products, and the like. The more common "low temperature" type are maintained at evaporator temperatures in the range of approximately minus 25.degree. F. to assure the frozen foods being properly kept. These display cases achieve refrigeration by the cooling phenomenon occurring with evaporation of a refrigerant such as Freon from a liquid state to a gaseous state. This is achieved by passing the liquid through an expansion valve, converting it to a gas as it flows into and through the evaporator coils in the display case where heat exchange occurs. The evaporated refrigerant gas is then compressed, causing heating, is cooled to remove this heat by passage through a condenser which liquifies the compressed gas refrigerant, and recycled back, usually through one or more storage vessels, to the expansion valve and evaporator. The condenser is cooled by air flow or liquid flow, usually the former.
Because the condenser is often located exteriorly of the building containing the display cases for air cooling by outside air, it is subjected to widely varying ambient air temperatures. Hence, the condensed refrigerant leaving the condenser will be cooled differing amounts depending upon the season. This causes the departing refrigerant pressure to vary widely. During winter months, especially in northern climates, cooling of the refrigerant can be so significant that the low pressure of the refrigerant leaving the evaporator can cause difficulties at the expansion valve. Hence, conventional practice is to install what are known in the trade as winter head pressure controls, at the condenser, to keep the pressure of the liquid refrigerant up to a certain minimum value. The purpose of such systems is to provide totally liquid phase refrigerant to the expansion valve which cannot tolerate any significant amount of refrigerant gas. Consequently, the refrigerant must be totally condensed to a liquid phase in the condenser, and held there until the compressor pumps the pressure up to a condition where the expansion valve can be guaranteed an all-liquid inlet refrigerant. Stated differently, the winter head pressure controls on present outdoor condensers never allow liquid-line pressures to drop below a predetermined value, usually approximately 165 PSIG for Refrigerants 22 and 502; and 100 PSIG for Refrigerant 12. In order to maintain such a pressure, the compressor is required to work over and above that necessary to produce the load refrigeration necessary for cooling. This requires very significant amounts of extra energy.